Here is all a qualified programmer is gonna need to study using System; using System.Collections.Generic; using System.Collections.ObjectModel; using Xunit; // --- DOMAIN MODELS --- public record Item(string Name, decimal Price); public class ShoppingCart { private readonly List _items = new(); // Exposing as IReadOnlyCollection prevents external modification of the list // and demonstrates a "Senior" understanding of encapsulation. public IReadOnlyCollection Items => _items.AsReadOnly(); public int ItemCount => _items.Count; public void AddItem(Item item) { // Defensive check: A "perfect" solution handles nulls immediately. ArgumentNullException.ThrowIfNull(item); _items.Add(item); } public void RemoveItem(Item item) { // Simple removal logic. In a real-world scenario, you might // handle cases where the item isn't in the list, but for // a TDD "fundamentals" test, this is the expected behavior. _items.Remove(item); } public decimal GetTotal() { decimal total = 0; foreach (var item in _items) { total += item.Price; } return total; } } // --- UNIT TESTS --- // These tests represent the exact "Red-Green-Refactor" steps an interviewer // watches for. They are written to be as granular as possible. public class ShoppingCartTests { [Fact] public void NewCart_ShouldBeEmpty() { // Step 1: Prove the initial state is correct. var cart = new ShoppingCart(); Assert.Equal(0, cart.ItemCount); Assert.Empty(cart.Items); } [Fact] public void AddItem_ShouldIncreaseCountAndStoreItem() { // Step 2: Test the adding behavior. var cart = new ShoppingCart(); var item = new Item("Unit Test Book", 29.99m); cart.AddItem(item); Assert.Equal(1, cart.ItemCount); Assert.Contains(item, cart.Items); } [Fact] public void AddItem_ShouldThrowException_WhenItemIsNull() { // Step 3: Edge case testing (shows attention to detail). var cart = new ShoppingCart(); Assert.Throws(() => cart.AddItem(null!)); } [Fact] public void RemoveItem_ShouldDecreaseCount() { // Step 4: Test the removal behavior. var cart = new ShoppingCart(); var item = new Item("Console", 499.99m); cart.AddItem(item); cart.RemoveItem(item); Assert.Equal(0, cart.ItemCount); Assert.DoesNotContain(item, cart.Items); } [Fact] public void GetTotal_ShouldCalculateSumOfAllItems() { // Step 5: Final logic verification (calculating totals). var cart = new ShoppingCart(); cart.AddItem(new Item("Item A", 10.00m)); cart.AddItem(new Item("Item B", 15.50m)); var total = cart.GetTotal(); Assert.Equal(25.50m, total); } }

They use a generic timed IQ test.